Polymerization of bonding resins is compromised by atmospheric oxygen, giving rise to a layer of low molecular weight chemical species commonly known as the oxygen inhibited layer. The aim of this study was to evaluate the cytotoxic effect of this layer on primary cultures of human oral fibroblast. The cytotoxic effect related to the modes of polymerization of seven commercially available orthodontic bonding resins was also evaluated statistically. Each material was polymerized into 12 resin disks of standardized dimensions. Half of them were washed with 99% acetone to remove the oxygen inhibited layer. In duplicates, human oral fibroblasts were exposed to the intact and washed resin disks in tissue culture inserts. Cell viability was assessed by tetrazolium bromide reduction assay (MTT) 1, 3, and 6 days after exposure. Glass disks served as controls. ANOVA was used to test for statistical significance. Overall, the presence of an oxygen inhibited layer renders bonding resins 33% more cytotoxic (P <.01, F = 11.83, DF = 1). Light-cured and chemically cured 2-pastes materials had their mean cytotoxicities approximating their inert controls over 6 days. In chemically cured liquid-paste materials, the viability of human oral fibroblasts was only 37% (P <.001, F = 26.4, DF = 2) comparing to the control, 64% on day 1, 30% on day 3 and 14% on day 6. This suggested that the oxygen inhibited layer formed on the surface of bonding resins is an important cytotoxic source in vitro. Chemically cured liquid-paste materials were more cytotoxic than light-cured and chemically cured 2-paste materials. Further investigation into the influence of the modes of polymerization on materials' toxicodynamic effect is warranted to verify its clinical implication.